The subject invention is directed to a sheet product which is useful in providing separator components for batteries and to improved batteries containing the formed separator. More specifically, the present invention is directed to a thin microporous sheet product composed of a highly filled polymeric matrix or a low-level filled (including unfilled) polymer matrix having a porous support sheet embedded between the sheet product's first and second major surfaces and having an average pore size distribution across the thickness of the sheet product such that there is a gradient of nominal pore size increasing from each major surface towards the interior central portion of the sheet product's thickness.
Storage batteries have at least one pair of electrodes of opposite polarity and, in general, have series of adjacent electrodes of alternating polarity. The current flow between these electrodes is maintained by an electrolyte which may be acidic, alkaline, or substantially neutral depending upon the nature of the battery system. Separators are located in batteries between adjacent electrodes of opposite polarity to prevent direct contact between the oppositely charged electrode plates while freely permitting electrolytic conduction. Separator components have taken many forms. In a modern battery design, the separator is in the form of a thin sheet or film or more preferably, a thin envelope surrounding each electrode plate of one polarity.
It is generally agreed that one of the critical elements in a battery design is the separator component and, to be highly effective in the design, the separator should have a combination of features. The battery separator must be resistant to degradation and instability with respect to the battery environment, including the other battery components and the battery chemistry. Thus, the battery separator must be capable of withstanding degradation of strong acids (such as sulfuric acid commonly used in acid battery designs) or strong alkali (such as potassium hydroxide used in alkaline battery designs) and to do so under ambient and elevated temperature conditions. Further, the separator should also be of a thin and highly porous character to provide a battery of high energy density. Although battery separators of thick or heavy design have been utilized in the past, such materials detract from the overall energy density of the battery by reducing the amount of electrodes that can be contained in a predetermined battery configuration and size. Another criteria is that the battery separator must be capable of allowing a high degree of electrolytic conductivity. Stated another way an effective separator membrane must exhibit a low electrical resistance when in the battery. The lower the electrical resistance the better the overall battery performance will be. A still further criteria is that the separator should be capable of inhibiting formation and growth of dendrites. Such dendrite formation occurs during battery operation when part of the electrode material becomes dissolved in the electrolyte and, while passing through the separator, deposits therein to develop a formation which can, after a period of time, bridge the thickness of the separator membrane and cause shorting between electrodes of opposite polarity.
In addition to meeting the above combination of properties, it is highly desired to have a sheet product which is capable of exhibiting good physical properties of tensile strength, flexibility and ductility to withstand the handling and processing without developing imperfections and cracks which would cause the sheet product to be unsuitable as a battery separator. Meeting this criteria is contrary to some of the above 10 described properties (i.e. thin and light weight material and high porosity to provide good conductivity). In providing envelope type separators, these physical properties must also be accompanied by the ability of the material to be sealable by heat and to able to be folded upon itself so as to provide a pocket separator design. As part of the physical property requirements, the sheet product should be capable of exhibiting a high degree of integrity during formation and use.
Various microporous membranes or sheet materials have been suggested for utilization as a battery separator. Separators conventionally used in present battery systems are formed of polymeric films which when placed in an electrolyte or an electrolyte system, are capable of exhibiting a high degree of conductivity while being stable to the environment presented by the battery system. The films include macroporous as well as microporous materials. The porosity permits transportation of the electrolyte. Examples of such separators include unfilled polyolefin sheets which have been stretched and annealed to provide microporosity to the sheet, such as is described in U.S. Pat. Nos. 3,558,764; 3,679,538; and 3,853,601. In addition, polyolefin separators which include filler materials are disclosed in U.S. Pat. Nos. 3,351,495 and 4,024,323. In general, such polymer/filler compositions are friable materials and tend to exhibit electrical resistance which does not permit the formation of a highly efficient, high energy battery system.
Polyvinyl chloride has been used in forming battery separators. The polymer is conventionally initially formed into small particles which are sintered and pressed into sheet material. The polymer has also been mixed with a liquid and a filler to form a doughy composition which can be kneaded into a sheet from which the liquid evaporates. Such separators are difficult to form, provide a product of high electrical resistance and have poor physical properties. Specifically, such separators are brittle and not capable of undergoing the various stress forces encountered in the battery formation and operation and of being folded upon itself to provide enveloped designed batteries. Polyvinyl chloride has been copolymerized as well as blended with other polymeric material (i.e. copolymers of vinyl chloride and acrylonitrile) in attempts to impart better properties to the resultant product. Although improved products are obtained, they do not provide the combination of properties described above.
It is highly desired to have a battery separator which is capable of exhibiting very low electrical resistance while at the same time providing the combination of properties described above.